Letting developer from another country access AWS

It is normal for users to have accounts and access in other regions. You can consider use cases where someone in the U.S, wants to deploy and support their application in Europe. Typically, this is not something that would cause issues. You may run into issues if access from China to an AWS region is blocked by China. This is not something AWS controls, rather this is controlled by the China government. AWS does have the ability to block access from known IP ranges (deny lists) that originate from DDoS or other hacking attempts, although that is the WAF denying access to your application endpoints. Let's say your developer is attempting to load test your application, and they are originating from a denied IP range or origin--that could be blocked. I suggest setting up the login and see if the developer can successfully log in.